In recent years, more precise accuracy in terms of optical element shape has been demanded along with miniaturizing and thinning of optical elements such as of digital cameras and portable equipment. That is, the technology has been progressing by which highly-precise processed lenses are highly precisely assembled. In such highly-precise optical systems, the effect on performance degradation due to temperature change is unlikely to be ignorable.
Among others, simulation on the basis of temperature analysis is essential for portable or in-car products that are assumed to be used under conditions of rapid temperature change in particular. The temperature analysis of conventional optical systems has been often investigated for exclusively highly-precise optical systems such as steppers in large part of semiconductor fields, etc.
In the new fields described above, however, there has been a growing necessity to consider refractive index change with temperatures of optical materials themselves, thermal expansion coefficient, and expansion coefficient of lens-supporting materials. Accordingly, such an optical material has been demanded that has lower thermal expansion coefficients and lower refractive index change with temperature change.
The temperature coefficient of refractive index, employed in temperature analysis of optical materials, is defined as dn/dT, which indicates a relation between temperature and refractive index. The temperature coefficient of refractive index depends on the measuring wavelength and its temperature region, and is expressed in terms of the temperature coefficient of relative refractive index in air at the same temperature with the glass (dn/dT relative, 101.3 kPa, in dry air) and the temperature coefficient of absolute refractive index in vacuum (dn/dT absolute).
Glasses, containing a large amount of La2O3 having features of lower dispersibility and higher transmittancy, etc., typically have a higher dn/dT and, those having lower values thereof are far from practical use. In a region of shorter wavelengths, dn/dT tends to change considerably and thereby degrades performance.
In general, absorption edges of optical materials tend to shift toward the longer wavelength side from UV region to the visible region as their refractive indices are higher. That is, glasses of higher refractive indices tend to deteriorate their transmissive properties at the region of shorter wavelengths. No substantial absorption is recognizable in the visible region of the longer wavelength side from absorption edges in conventional optical glasses, and therefore those having proper transmissive properties at the near-UV region typically have proper light transmittancy in the visible region. In addition, the UV ray of wavelength 400 nm or visual light near the UV ray have been widely applied to input information to optical magnetic recording media in order to increase memory capacity. Accordingly, the requirement for glass having higher UV transmissive properties is very high.
When used for lenses to project UV rays, astrometric telescopes, etc., materials are also demanded that have solarization resistance.
In recent years, the technology to save weight or miniaturize optical elements has been applied to many products by way of making use of aspheric surface produced by mold-press shaping optical elements of portable equipment such as digital cameras and cellular telephones. However, when the aspheric surface is to be obtained by conventional grinding and polishing processes, many highly expensive and complicated operating processes are necessary. Therefore, such a method is employed that directly shapes lenses, using an ultra-precisely finished mold tool, from preform materials that are produced by way of dropping molten glass or grinding and polishing plate glass. The lenses can be produced by such a method at lower cost and with quick delivery. The molding method, referred to as glass mold, has been thoroughly researched and developed, and as a result, aspheric lenses by the glass mold employed in optical equipment tend to increase year by year.
For these glasses, low-temperature softening glasses are required for the employed glass in view of heat resistance of mold tools used in the glass mold. However, the Tg of conventional glasses, containing SiO2, B2O3, and La2O3, is typically above 600° C., and thus glass satisfactory for the heat resistance of press molds has not existed heretofore.
In regards to optical glasses containing SiO2, B2O3, and La2O3, Patent Document 1 discloses an optical glass for precision press lenses that contains F (fluorine) as an essential component. In addition, Patent Document 2 discloses an optical glass that contains SiO2, B2O3, ZnO, and La2O3 as its essential components.
Patent Document 1: Japanese Examined Patent Application Publication No. 2738744
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-161506
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-161506
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2003-201143